An electrophotographic printer forms an electrostatic latent image on a photosensitive drum by light according to an image signal, develops the latent image by selectively attracting toner thereto, and then transfers the developed image onto a paper to obtain a print image. As a light source for forming the electrostatic latent image, a laser and a light-emitting diode array are widely used. In particular, since a light source constituted by the light-emitting diode array does not need a long optical path unlike the laser-type light source, it is suitable for small-sized printers. Because the light-emitting diode array can be laterally long, it is also suitable for large-sized printing. Demand has recently been mounting on light-emitting diode arrays of higher precision, higher light output and low in cost, as printing has been becoming faster with higher image quality, and as printers have been becoming smaller.
To achieve the reduction of the cost of light-emitting diode array heads, it is advantageous to replace a static driving system comprising ICs for separately driving light-emitting diodes, with a dynamic driving system or a matrix driving system, which comprises pluralities of light-emitting diodes in a block, and a switching matrix wiring subjected to time division operation, thereby reducing the number of driving ICs and bonding wires.
One of effective means for achieving further cost reduction in the production of a dynamic-driving light-emitting diode array seems to increase the number of LED array chips (referred to simply as “LED chips”) per a wafer by reducing the size of the LED chips. However, because the LED chips should be arranged in numbers corresponding to a printing size in an LED head, the reduction of the size of the LED chips in a longitudinal direction results in increase in the number of the LED chips arranged, failing to achieve cost reduction. Accordingly, the miniaturization of the LED chips could be achieved only in a chip width direction.
The reduction of the width of an LED chip may be achieved by (i) the reduction of the size of bonding pads occupying most of the LED chip, or by (ii) the arrangement of first bonding pads connected to cathodes via common wirings and second bonding pads connected to anodes in a row on one side of the LED chip. Though both of the measures (i) and (ii) are optimally taken, particularly the measure (i) disadvantageously increases mass production cost, because there is a limit in the miniaturization of bonding pads per se, and because bonding apparatuses for small bonding pads are expensive. On the other hand, with respect to the measure (ii), it is difficult to provide each bonding pad with sufficient area, when the precision of the LED head is as high as 600 dpi, particularly 1200 dpi. The bonding pads in the LED chip generally need to be as large as 60 to 80 μm in a chip width direction (the longitudinal direction of the LED chip) and 80 to 120 μm in a longitudinal direction (the transverse direction of the LED chip).
One example of a light-emitting diode array of a four-partitioned dynamic driving system is shown in FIGS. 8 and 9. The light-emitting diode array of this example comprises a substrate 10, a conductive layer 11 formed on the substrate 10, pluralities of separate light-emitting portions 1 formed on the conductive layer 11, cathodes 2 each formed on an upper face of each light-emitting portion 1, anodes 3 formed on the conductive layer 11, four common wirings 4 separately connected to the cathodes 2 via lead wires 5c, bonding pads 6c separately connected to the common wirings 4, and bonding pads 6a separately connected to anodes 3 via lead wires 5a. As shown in FIG. 8, the bonding pads 6c, 6a are arranged in a row such that the ratio of the number of the bonding pads 6c to the number of the bonding pads 6a is 1:1. The light-emitting portions 1 are partitioned by a first rectangular groove 20 formed in the conductive layer 11 to blocks such that each block comprises four light-emitting portions 1.
The total number of the bonding pads 6c, 6a is half of the number of the light-emitting portions 1 in the LED chip. For instance, in a light-emitting diode array of 1200 dpi having a dot pitch of 21.2 μm, the width of each bonding pad should be less than 42.4 μm, less than a minimum size (60 μm) of each bonding pad.
To solve such problem, a light-emitting diode array comprising eight dots of diodes subjected to time-division driving via eight common wirings is disclosed (JP 2001-77431 A, and “Oki Technical Review,” Vol. 69, No. 1, January, 2002, Serial No. 189). With an 8×8 multi-layer matrix wiring comprising eight common wirings as described therein, the number of bonding pads can be reduced to half, and thus a design margin in a chip width direction can be doubled to less than 84.8 μm. Increase in the number of common wirings, however, leads to increase in the width of an LED chip.